The invention relates to the use of scale inhibitors in the presence of hydrogen fluoride. More specifically, it relates to making scale inhibitors that previously could not be used in the presence of hydrogen fluoride useful in the presence of hydrogen fluoride.
Five principal generic types of inorganic scale inhibitors are commercially available. They are: phosphonates, phosphino-polycarboxylates, polyacrylates, poly vinyl sulphonic acids, and sulphonated polyacrylate co-polymers. Because of the pKa values of the corresponding phosphonic and phosphino-polycarboxylic acids, most phosphonates and phosphino-polycarboxylate scale inhibitors are ineffective at pH values below about 4; they do not dissociate to a sufficient extent at lower pH's. Two methods have commonly been used to address this issue. One method has been to increase the pH (at least partially neutralize the solution) in order to increase the extent of dissociation of the corresponding acids. The other has been to increase the electrostatic attraction of the inhibitor molecules by introducing strongly acidic sulphonic acid groups.
In addition, simultaneous (or sequential) treatment that includes both scale control and matrix acidizing has become more and more attractive due to its cost-effectiveness and operational simplicity. In order to increase the effectiveness of the scale inhibitor, the pH of the inhibitor is generally adjusted high enough (sometimes to greater than 7) to compensate for the pH drop that may occur if there is contamination of the inhibitor by the acid. This is done by adding caustic additives, such as NaOH, Ca(OH)2 or others; there may also even be addition of NaCl/CaCl2 or other salts (to promote precipitation of the inhibitor).
It is well-known that sandstone acidizing fluid systems (HF-based, containing HF or HF sources such as ammonium fluoride or ammonium bifluoride) are incompatible with cations such as Na+, K+, and Ca++ in aqueous solution. If concentrations reach certain levels, NaF and similar fluoride salts precipitate.
There is a need for scale inhibitors that can be used in situations in which they may come into contact with fluoride ions.